Method for starting a hybrid drive

ABSTRACT

A method is described for starting a hybrid drive having a battery ( 1 ), an internal combustion engine ( 9 ), and at least two electric machines ( 5, 8 ), at least one being used to start the internal combustion engine ( 9 ). Reliable starting even at very low temperatures is achieved by bringing at least one of the electric machines ( 5 ) to a specified or specifiable rotational speed in a state that is uncoupled from the internal combustion engine ( 9 ) at the beginning of a starting phase, while electric power is supplied from the battery ( 1 ), and the electric machine is subsequently switched to generator operation; the electric power generated in generator operation is supplied to at least one further one of the electric machines ( 8 ) which then starts the internal combustion engine ( 9 ).

BACKGROUND INFORMATION

[0001] The present invention relates to a method of starting a hybrid drive which has a battery, an internal combustion engine, and at least two electric machines, at least one being used to start the engine.

[0002] In motor vehicles today, the internal combustion engine is started by a conventional starter. This starter and its connection to the crankshaft are implemented so that the starting torque required for starting the engine is applicable at any time, even at extremely low temperatures (less than −10° C.). The electric current required for this is usually supplied by a 12-volt lead-acid battery.

[0003] Motor vehicles having a hybrid drive, as described in PCT/DE99/03026 and in German Patent Application 197 39 906 A1, have one or more electric machines, which may be operated either as motors or as generators, in addition to the internal combustion engine.

[0004] A conventional starter is usually omitted in such a hybrid drive. Electric machines in the vehicle assume the function of starter. Since the connection of the electric machines to the internal combustion engine is not optimized for starting the engine, these electric machines require a high electric current from the battery. Although the battery of a hybrid vehicle is designed for a much higher power level than the battery in a conventional vehicle, batteries are frequently unable to meet the electric power demand at cold temperatures in particular.

[0005] The object of the present invention is to provide a method of starting a hybrid drive of the type defined in the preamble, so that reliable starting of the internal combustion engine is ensured even at low temperatures.

[0006] This object is achieved through the features of claim 1, according to which at the beginning of a starting phase, at least one electric machine is made to rotate at a specified or specifiable rotational speed, while uncoupled from the engine, and then is switched to generator operation; the electric power generated in generator operation is supplied to at least one of the electric machines and is used to start the internal combustion engine.

[0007] When uncoupled from the internal combustion engine during the starting phase, the particular electric machine may be brought to a very high rotational speed using the battery power available even at low temperatures, and this may be accomplished relatively rapidly (e.g., within a few seconds). Then in subsequent generator operation, the electric power generated by this (these) electric machine(s) is advantageously available for starting the engine in addition to the electric power supplied by the battery.

[0008] An advantageous measure includes the fact that in the starting phase of the engine, the voltage generated by the at least one electric machine is regulated at a constant level. A voltage dip at the battery is regulatable through generator operation of the particular electric machine(s), so that the additional electric machine(s) running in motor operation to drive the internal combustion engine is (are) able to supply its (their) maximum torque.

[0009] For starting the engine at the highest possible torque, it is advantageous that the at least one additional electric machine is additionally supplied with electric power from the battery.

[0010] The starting operation of the internal combustion engine is also supported to advantage by the fact that a decelerating torque of the at least one electric machine during generator operation is used to start the engine.

[0011] If the rotational speed of the at least one electric machine is specified as a function of a battery condition detected, then the starting phase may be shortened as a function of a particular battery condition, e.g., by not accelerating the electric machine(s) provided for generator operation to the highest possible rotational speed.

[0012] With the measures indicated here, the energy from the battery is first stored temporarily in the form of kinetic energy in at least one electric machine before the engine is started. The stored kinetic energy is used for power generation in a short period of time in startup in that the at least one particular electric machine switches to generator operation. Due to the conversion of kinetic energy into electricity in at least one electric machine, a sufficiently high on-board power is made available within a short period of time even at very low temperatures in starting the engine. An additional advantage is the heating of the battery during acceleration of the at least one electric machine to be operated in generator operation, so that more power can be drawn from the battery in starting up the engine using the at least one other electric machine.

[0013] The present invention is explained in greater detail below on the basis of one exemplary embodiment, with reference to the drawing.

[0014]FIG. 1 shows a schematic diagram of the principle of cold start of a hybrid drive;

[0015]FIG. 2 shows a flow chart for startup of the hybrid drive;

[0016]FIG. 3 shows a curve of the rotational speeds of the electric machines and the internal combustion engine; and

[0017]FIG. 4 shows a schematic diagram of an essentially known hybrid drive, in which the starting method according to the present invention is implementable.

[0018]FIG. 1 illustrates the principle of a cold start on the example of a hybrid drive having an internal combustion engine 9 and two electric machines 5, 8. A battery 1 is connected by a cable 2 to control units 3, 6 of electric machine 5 and to additional electric machine 8. Control units 3, 6, which may be designed as pulse-width-modulation inverters, for example, trigger electric machines 5, 8 via suitable lines 4, 7. Setpoint values, e.g., rotational speeds or torques, may also be preset by an external control unit (not shown) over appropriate feeder lines 12, also taking into account a monitoring device for the battery power. One electric machine 5 may be kinematically uncoupled from engine 9 with the help of an uncoupling stage 10, e.g., a clutch, to bring it to a specified or specifiable rotational speed first in motor operation and then to operate it in generator operation, and to make available the triggering current for additional electric machine 8, which is connected to engine 9 via transmission 13. In addition to the generator current of electric machine 5, additional electric machine 8 must receive the highest possible electric power from the battery to achieve the highest possible torque of additional electric machine 8 for starting engine 9.

[0019] In the flow chart shown in FIG. 2 for starting the engine, first clutch 4 is opened and electric machine 5 is accelerated to its maximum rotational speed, for example, as rapidly as possible and at the maximum battery power. In doing so, electric machine 5 is operating in motor operation. As soon as electric machine 5 has reached the specified rotational speed, it switches to generator operation and is regulated at a constant voltage level. Then additional electric machine 8 is triggered to start the engine at the highest possible torque. A voltage dip which then occurs across the battery is adjusted by electric machine 5 working in generator operation, so that additional electric machine 8 can still deliver its maximum torque. In generator operation of electric machine 5, the previously stored kinetic energy is converted into on-board electric power. For storing kinetic energy, connected transmission stages and a countershaft may also be used in addition to the rotor.

[0020] In a hybrid drive having a dual-E transmission, such as that known from the related art cited in the preamble and illustrated in FIG. 4, the starting operation proceeds as follows, for example:

[0021] No gear is engaged on a countershaft S. Countershaft S and thus also electric machine 5 are able to rotate freely.

[0022] Any gear is engaged on another countershaft L, and the parking brake is set or the wheel brake is activated. Countershaft L is thus blocked.

[0023] Electric machine 5 is then brought to a sufficiently high rotational speed with the available current. Kinetic energy is stored in the inertia of the rotor, the transmission stages and countershaft S.

[0024] Electric machine 5 is then decelerated at the maximum generator torque. The rotor of electric machine 5 and thus also the countershaft S are decelerated. The decelerating torque on countershaft S generates a starting torque in the engine with the transmission ratio of a planetary gear P2, thus making an additional contribution to the electric power.

[0025] The electric current of electric machine 5 is utilized to drive additional electric machine 8, which thus acts as a motor to accelerate the internal combustion engine. The maximum possible battery current is utilized here to advantage.

[0026]FIG. 4 also shows a crankshaft KW, an output shaft AW and another planetary gear P1 plus other transmission parts that are not shown in detail.

[0027]FIG. 3 shows a curve of the rotational speed of the two electric machines 5, 8 and the internal combustion engine over time, as obtained with the starting operation described here. First the rotational speed of electric machine 5 is brought to a predetermined rotational speed, e.g., the maximum rotational speed, by the available battery power, and then electric machine 5 is switched to generator operation at a point in time t1. The electric power generated by electric machine 5 in generator operation is sent to additional electric machine 8 to start the internal combustion engine. The engine is started again at a point in time t2. 

What is claimed is:
 1. A method of starting a hybrid drive comprising a battery (1), an internal combustion engine (9), and at least two electric machines (5, 8), at least one of which is used to start the engine (9), wherein at the beginning of a starting phase, at least one of the electric machines (5) is brought to a specified or specifiable rotational speed in a state that is uncoupled from the internal combustion engine (9), while electric power is supplied by the battery (1), and then [the electric machine(s) is (are)] switched to generator operation, and the electric power generated in generator operation is supplied to at least one further one of the electric machines (8) which then starts the internal combustion engine (9).
 2. The method as recited in claim 1, wherein the voltage generated by the at least one electric machine (5) is regulated to a constant level in the starting phase of the internal combustion engine (9).
 3. The method as recited in claim 1 or 2, wherein the at least one additional electric machine (8) is additionally supplied with electric power from the battery (1).
 4. The method as recited in one of the preceding claims, wherein, a decelerating torque of the at least one electric machine (5) is used as well during generator operation to start the internal combustion engine (9).
 5. The method as recited in one of the preceding claims, wherein the rotational speed of the at least one electric machine (5) is specified as a function of a detected battery state. 